Remember that in Galera, replication beats write locks.  If you want to avoid the deadlock ensure that updates to the table happen from only one node, otherwise you simply have to retry the transaction.  this is the case with Galera or regular innodb.  Your application must be able to handle deadlocks.

You must make sure there is an index on (user_id,domain) or you will still lock the table.

So either send all SQL that changes data through a single node (you can still failover to another node for HA) or deal with deadlocks, though in reality you have to deal with deadlocks in either case.

--Justin